1. Field of the Invention
The present invention generally relates to Charged Particle Beam Projection Systems, and more particularly, to a method for fabricating high precision, thermally stable electromagnetic coil vanes used with Charged Particle Beam Projection Systems.
2. Background Description
Yokes and electromagnetic lenses are widely used in charged particle beam tools such as electron beam tools, electron microscopes, and cathode ray tubes and ion beam systems. Yokes and lenses employing toroidal magnetic deflection coils are commonly used in electron beam lithography systems for focusing an electron beam onto a substrate for submicron patterning of semiconductor devices.
U.S. Pat. No. 4,251,728 to Pfeiffer shows an example of a toroidal magnetic deflection yoke. FIG. 1 shows a top view of a traditional toroidal yoke similar to that shown in Pfeiffer. The traditional yoke includes a plastic form having slots numbered from 1 to 20, and forms both X and Y coil axes.
Presently, electromagnetic coils are formed by winding wire into multiple radial grooves cut into a plastic form. Also, deflection yokes have been made from round wire, with bondable insulative coatings. The method used for winding the form to make the coils requires alternating between the X and the Y windings. As the number of radially cut grooves and the number of turns increases, so does the degree of difficulty and the time involved for winding the form to make the coils.
Ser. No. 09/325,162 by Rodney Kendall and David Pinckney for xe2x80x9cFabrication Method of High Precision, Thermally Stable Electromagnetic Coil Vanesxe2x80x9d (Nikon Corporation Docket NC98-591) discloses a method and apparatus for fabricating electromagnetic coil vanes having two complementary patterns of approximately 22-23 American Wire Gauge (AWG) uninsulated rectangular cross section wire to be accurately bonded to a thin substrate. The wires preferably are made from thin sheets of copper, via wire electrical discharge machining (EDM), which enables many coils (preferably 50 or more) to be cut from stacked copper sheets, clamped together. The wire cross-section produced by this technique is rectangular with vertical sidewalls. This method requires specialized tooling to mechanically align and clamp the machined coils to the substrate, and a bonding composition that adheres the coils to the substrate.
U.S. Pat. No. 5,100,714 to Zsamboky describes a process of forming a thin copper film which is bonded to a ceramic substrate. This copper film is then patterned via photolithography and plated to build up the copper thickness. U.S. Pat. No. 5,733,468 to Conway describes similar photolithography steps used to produce an image in thin copper and then plating to increase the copper thickness. The initial thin copper is in a foil form. U.S. Pat. No. 5,454,927 to Credle et al. describes a method to fill via holes in a ceramic substrate. The methods disclosed in these patents are not used for devices whose object is to carry substantial current, but instead are used for fabricating printed circuits and like electronic devices.
It is therefore an object of the invention to provide a method for fabricating high precision, thermally stable electromagnetic coil vanes that provides a more accurate coil vane pattern placement.
It is another object of the invention to provide a method for fabricating high precision, thermally stable electromagnetic coil vanes that does not require specialized tooling to align and clamp the copper coils.
It is yet another object of the present invention to provide a method for fabricating high precision, thermally stable electromagnetic coil vanes that does not require the use of a bonding composition to bond the copper coils to the substrate.
According to the invention, a method is provided to fabricate high current capacity, high accuracy, thermally stable deflection yokes used to generate off axis beam deflections.
In the preferred embodiment, electromagnetic coil vanes are formed having two complementary patterns of approximately 25-28 AWG uninsulated rectangular cross section copper wire. The copper wire is formed by accurately depositing the copper on opposing sides of a thin substrate, and subsequently bonding the copper to the substrate. Preferably, the coefficient of thermal expansion (CTE) of the thin substrate should be low. For example, in a preferred embodiment, ceramic can be used, which has a CTE of approximately 7xc3x9710xe2x88x926/xc2x0 C. The two complementary copper coil patterns are electrically connected by a plated through hole. The gaps between adjacent copper wires must be as small as possible to maximize the copper wire pattern density and thus the effectiveness of the coil.
The method involves photolithographically exposing high resolution, dense wire patterns in a flash coat of copper, on both sides of a ceramic vane substrate. Additional copper is then deposited on both high resolution patterns and in the through hole by plating until the desired thickness of copper is obtained. Preferably, the copper is deposited simultaneously on both sides of the substrate. Then a firing operation is performed that bonds the copper wires to the substrate without the use of a bonding composition.